For example, along with tendency toward high functionality and large capacity of electronic devices, generation of heat is increasing, and a material having a high thermal conductivity and a low coefficient of thermal expansion useful for heat removal has been required. Most of heat generated from an electronic circuit comprising a semiconductor device, a resistor, a transformer, a condenser and a wiring is transmitted from a circuit board or a base substrate as a support for the circuit board to a cooling device and is finally released to the air or a cooling liquid. In an electronic circuit which generates a large quantity of heat, aluminum, copper or an alloy thereof having good thermal conductivity is usually used for the material of the base substrate.
Further, in recent years, a heat-transfer material having a coefficient of thermal expansion adjusted low by combining carbon fibers or a ceramic with a metal has been proposed (for example, Patent Document 1). However, although aluminum, copper or an alloy thereof used for the material of the base substrate has good thermal conductivity, it has a large coefficient of thermal expansion in its nature. Whereas, an electronic circuit comprising a ceramic or a silicon semiconductor device to be laminated on the base substrate has a low coefficient of thermal expansion, and accordingly by the difference in thermal expansion between them, warpage, peeling or the like may occur.
As a material which solves the above problem, a substrate made of a composite material comprising silicon carbide, alumina, silicon nitride or aluminum nitride as a ceramic having a low coefficient of thermal expansion and alumina or copper metal, has been proposed. However, this composite material substrate has such a drawback that its processing is difficult since it contains a ceramic. Further, a substrate comprising silicon and an aluminum alloy has also been proposed but has not been used practically yet.
As mentioned above, heretofore, proposed materials may satisfy both thermal conductivity and coefficient of thermal expansion, but no material with favorable processability has been realized yet. Further, many of conventional materials have a high elastic modulus, and accordingly when materials differing in the coefficient of thermal expansion are bonded, a large thermal stress is applied to the bonding surface and as a result, peeling or the like may occur.
Patent Document 1: JP-A-11-97593